1. Field of the Invention
This invention relates to an electronically controlled power steering apparatus which controls the steering assist amount of a steering mechanism of a vehicle, and more particularly to an electronically controlled power steering apparatus of the type wherein the aimed assist amount is set in accordance with a fuzzy rule.
2. Description of the Related Art
In recent years, power steering apparatus have varied widely for assisting the force (hereinafter referred to as steering wheel operating force or steering force) for operating a steering wheel. Particularly, hydraulic power steering apparatuses which make use of a hydraulic cylinder mechanism to hydraulically assist the steering wheel operating force are popularly employed as such power steering apparatuses. Also, electrically operated power steering apparatuses wherein the steering wheel operating force is assisted by an electric motor, have been developed.
Such power steering apparatus as described above allow steering of a vehicle, for which high steering wheel operating force is required, such as, for example, a large size vehicle or a vehicle which employs wide tires for wheels for steering operation, to be performed with a low steering wheel operating force, eliminating so-called heavy operation of the steering wheel.
Another consideration is that, when the vehicle speed is low such as upon garaging, generally the steering operation is desired to be performed with a lower steering force. On the other hand, when the vehicle is running at a high speed, if the steering operation is very light, then running of the vehicle becomes unstable, and accordingly, the steering operation is desired to be rather heavy. Thus, a vehicle speed responsive power steering apparatus has been developed wherein the steering wheel operation is controlled in response to the speed of the vehicle such that, when the vehicle runs at a low speed, the steering assist amount is set to a comparatively high value so as to make the steering wheel operation lighter, but when the vehicle runs at a medium or high speed, the steering assist amount is set to a comparatively low value to make the steering wheel operation heavier.
In one of such vehicle speed responsive power steering apparatus, a vehicle speed sensor is provided on the vehicle while a valve for adjusting hydraulic oil to be supplied to a power cylinder is provided in a hydraulic system of the hydraulic power steering apparatus, and operation of the valve is controlled in response to a vehicle speed detected by the vehicle speed sensor to adjust the steering assist amount. The vehicle speed responsive power steering apparatus of the type just mentioned is called electronically controlled power steering apparatus.
In the following, construction of an exemplary conventional electronically controlled power steering apparatus will be described with reference to FIGS. 9 to 11.
Referring to FIGS. 9 to 11, an input shaft 11 is fitted for rotation in a casing 25 by means of bearings and is connected to receive steering force from a steering wheel not shown. A pinion 12 is mounted for relative rotation at a lower end of the-input shaft 11 with a bush or a like element not shown interposed therebetween.
A torsion bar 15 is located in the hollow inside of the input shaft 11. The torsion bar 15 is coupled at an upper end thereof for integral rotation to the input shaft 11 by way of a pin or a like element while it is not restrained at a lower end thereof by the input shaft 11.
The pinion 12 is held in serration coupling engagement with the lower end of the torsion bar 15 so that the steering force inputted to the input shaft 11 may be transmitted to the pinion 12 by way of the torsion bar 15. The pinion 12 is held in meshing engagement with a rack 13 so that the steering force may be transmitted to the rack 13 by way of the pinion 12 to move the rack 13 in its axial direction (in a direction perpendicular to the plane of FIG. 9) to steer wheels of the vehicle (not shown).
A power steering hydraulic cylinder 14 includes a cylinder 14A mounted on a member on the body of the vehicle, and a piston 14B provided intermediately of the rack 13 for movement in an axial direction in the cylinder section 14A together with the rack 13. The inside of the cylinder 14A is partitioned leftwardly and rightwardly by the piston 14B into a pair of oil chambers 14C and 14D.
A rotary valve 16 is provided for driving the hydraulic cylinder 14. Operating oil is supplied into or discharged from the left or right oil chamber 14C or 14D of the hydraulic cylinder 14 by opening or closing motion of the rotary valve 16 to provide steering assist force to the rack 13.
The rotary valve 16 is interposed between the input shaft 11 side and the pinion 12 side and is opened or closed in response to a difference in phase between the input shaft 11 and the pinion 12. In particular, when steering force is inputted to the input shaft 11, the input shaft 11 is rigid and presents little distortion, but the torsion bar 15 transmits the steering force to the pinion 12 while presenting some distortion, and consequently, the pinion 12 presents a difference in phase (angular position) to the steering side as compared to the input shaft 11. The rotary valve 16 is opened or closed so as to produce required steering assist force in the steering direction in response to the difference in phase.
A plurality of reactive force plungers 17 for providing, upon steering, steering reactive force to increase the steering force (that is, steering reaction) are provided on an outer periphery of a lower portion of the input shaft 11 such that they surround the outer periphery of the input shaft as seen in FIG. 11. The reactive force plungers 17 receive, at chambers 17A at back portions thereof, hydraulic oil supplied thereto under the control of a hydraulic pressure control valve 18 to restrain the input shaft 11 to provide steering reactive force in response to the hydraulic pressure. The chambers 17A communicating with an oil reservoir 24 by way of respective return orifices 22.
The hydraulic pressure control valve 18 is provided next to and extends in parallel to the input shaft 11 in the casing 25 as shown in FIG. 10 and includes a plunger 18A fitted for upward and downward sliding movement in the casing 25, a solenoid 19 for exerting upward axial force to the plunger 18A, and a spring 20 for normally biasing the plunger 18A downwardly.
The plunger 18A has a pair of oil passages and 18C communicating with the oil reservoir 24, an annular oil passage 18D for communicating with the oil pump 23, another annular oil passage 18F for communicoating with the chambers 17A of the reactive force plungers 17, and an oil passage 18F for communicating the annular oil passages 18D and 1BE with each other. Operating oil of a high pressure from the oil pump 23 is supplied from the annular oil passage 18D into the chambers 17A of the reactive force plungers 17 by way of the oil passage 18F and the annular oil passage 18E.
Upon steering, for example, while the vehicle stops or is running at a low speed, maximum current is supplied to the solenoid 19. Consequently, the plunger 18A is moved upwardly to its highest position in which the annular oil passage 18D is not communicated with the oil pump 23 and supply of oil to the chambers 17A of the reactive force plungers 17 is stopped. Consequently, the reactive plungers 17 do not restrain the input shaft 11, and steering can be performed with light force.
On the other hand, for example, while the vehicle is running at a medium or high speed, the current supply to the solenoid 19 is decreased in response to an increase of the vehicle speed. Consequently, when the steering wheel is at its neutral position, the axial force of the plunger 18A decreases as the current decreases, and as the axial force decreases, the plunger 18A is moved down so that the annular oil passage 18D is communicated with the oil pump 23 to allow oil to be supplied to the chambers 17A of the reactive force plungers 17.
In this condition, tile reactive force plungers 17 restrain the input shaft 11, and consequently, the steering wheel is held at its neutral position. Then, if the steering wheel is moved slightly from the neutral position, then the output of the oil pump 23 tries to rise. In this instance, the discharging pressure of the oil pump 23 acts upon the chambers 17A of the reactive force plungers 17 almost without being controlled by the hydraulic pressure control valve 18. Accordingly, in the proximity of the neutral position of the steering wheel, the steering force is increased and a sufficient response of the steering wheel at the neutral position is obtained, resulting in a feeling of stability of the steering wheel in the neutral position.
Upon steering while the vehicle is running at a medium or high speed, the output of the oil pump 23 rises, within an ordinary steering range, to increase the steering assist amount in response to steering of the steering wheel, that is, in response to an increase of the steering force. Meanwhile, the discharging pressure of the oil pump 23 acts upon the chambers of the reactive force plungers 17 while being controlled by the hydraulic pressure control valve 18. Accordingly, the reactive force plungers 17 act to restrain the input shaft 11 to increase the steering response (steering force).
AS a result, upon steering when the vehicle runs at a medium or high speed, the steering force required is increased by an amount corresponding to the action of the reactive force plungers 17 as compared with the steering force required when the vehicle stops or is running at a low speed. In short, the steering response is increased and a stable steering feeling is obtained at medium or high speed. Particularly, when the current supply to the solenoid 19 is decreased in response to an increase of the vehicle speed, as the speed of the vehicle increases, the steering assist amount decreases and the steering force (steering response) increases, and consequently, a steering feeling of higher stability can be obtained.
The steering assist characteristic can be controlled by adjusting the current to be supplied to the solenoid 19 in this manner. As seen from FIG. 11, the current to be supplied to the solenoid 19 a control unit (control means) 30 in response to, in addition to vehicle speed information from a vehicle speed sensor 31, for example, mode setting information from an EPS (electronically controlled power steering) mode changeover switch 32 and an engine rotation signal from an engine speed sensor 33 or a like element to control the solenoid 19.
In particular, the EPS mode changeover switch 32 can selectively set a normal mode and a sport mode in which the steering force is increased at a lower speed than in the normal mode. When one of the modes is set, the control unit 30 controls the steering assist characteristic of the power steering mechanism in accordance with the set mode. For example, when the sport mode is set, the current to be supplied to the solenoid 19 is adjusted in response to vehicle speed information so as to present such an assist characteristic that the assist amount decreases gradually as the vehicle speed increases from a medium speed range of the speed V.sub.1 as seen from FIG. 12. On the other hand, when the normal mode is set, the current to be supplied to the solenoid 19 is adjusted in response to vehicle speed information so as to present such an assist characteristic that the assist amount decreases gradually as the speed vehicle increases from a little higher speed range of the speed V.sub.2 (&gt;V.sub.1).
Further, when trouble in a detection circuit is detected from vehicle speed information, an engine rotation signal or the like, the solenoid 19 is switched off to effect fail-safe control.
A further consideration is that, when a driver (steering operator) becomes fatigued, the driver feels tired in operation of the steering wheel and feels like operating the steering wheel with lighter-force,
However, with the conventional electronically operated power steering apparatus, since control of the steering assist amount is performed merely in response to the vehicle speed as described above, the steering assist force is set irrespective of the fatigue condition of the driver.
Thus, a power steering apparatus wherein the steering assist amount is varied in response to the fatigue condition of a driver has been proposed and is disclosed in Japanese Utility Model Laid-Open Application No. Heisei 3-60480. In the power steering apparatus, the driving time of the vehicle is detected, and when the driving time is excessively long, it is determined that the driver becomes fatigued, and the steering assist amount is increased.
With that power steering apparatus, however, since the steering assist amount is increased as the driving time increases, also when the steering angle is particularly large and a medium or high lateral acceleration acts upon the vehicle as at a curved road on a mountain or the like, the steering force is generally light. Consequently, the response feeling in operation of the steering wheel becomes insufficient, and insufficient steering linearity or loss or absence of steering information is invited, resulting in the problem that the controllability or the tracing performance of the vehicle is degraded.
Further, when the driver takes a rest while the engine is operating, although the driver has recovered from the fatigue by the rest, the steering force remains light and the, driver may feel uncertain due to insufficient feeling or response in steering.
In short, since the steering assist amount is varied irrespective of a variation of the actual fatigue condition of the driver as the driving time passes, the power steering apparatus cannot accomodate the actual fatigue condition of the driver accurately to obtain an optimum steering assist amount.